1. Field of the Invention
The present invention relates to a power supply apparatus, and especially relates to a power supply apparatus with an auxiliary winding switching circuit.
2. Description of the Related Art
A power supply apparatus supplies power to an electronic apparatus to drive the electronic apparatus. The electronic apparatus informs the power supply apparatus of a required voltage (for examples, 5 volts or 19.5 volts) if the power supply apparatus supplies power through a universal serial bus interface.
The power supply apparatus comprises a transformer and a pulse width modulation controller. The transformer comprises a primary side winding, an auxiliary winding and a secondary side winding. The pulse width modulation controller is configured to change a voltage of the primary side winding when the electronic apparatus informs the power supply apparatus of the required voltage. The secondary side winding induces the voltage of the primary side winding to generate a voltage to send to the electronic apparatus. At the same time, the auxiliary winding induces the voltage of secondary side winding to generate a voltage to send to the pulse width modulation controller to supply power to the pulse width modulation controller.
When the required voltage is 5 volts, the voltage generated by the auxiliary winding is not too high, so that the pulse width modulation controller can utilize the voltage generated by the auxiliary winding directly. When the required voltage is 19.5 volts, the voltage generated by the auxiliary winding is too high, so that the pulse width modulation controller cannot utilize the voltage generated by the auxiliary winding directly. In an embodiment, the voltage generated by the auxiliary winding is 20 volts when the required voltage is 5 volts. The voltage generated by the auxiliary winding is 78 volts when the required voltage is 19.5 volts.
A regulating circuit is arranged to decrease the voltage generated by the auxiliary winding to protect the pulse width modulation controller when the voltage generated by the auxiliary winding is too high. However, the energy dissipation of the power supply apparatus is increased when the voltage generated by the auxiliary winding is decreased by the regulating circuit.